Steel strip can be annealed in either coil form, commonly referred to as "batch annealing" or by passing the uncoiled strip through a continuous annealing furnace. The present invention addresses itself to the continuous annealing of steel strip. Continuous annealing furnaces are of two types, namely, vertical or horizontal. Vertical or horizontal refers to the strip alignment in the furnace during the heat treatment operation. The major affect on product quality in comparing the two types is the capability of the horizontal furnace to produce a stress-free strip product.
A typical horizontal continuous heat-treating furnace roller hearth line consists of an entry section including coil-payoff reels, strip welder, horizontal looping equipment, electrolytic cleaning, scrubbing and drying units. The entry section is joined to a contiguous horizontal heat-treating section consisting of a gas-fired heating zone (72 feet long), and an electrically-heated holding zone (600 feet long), a controlled cooling zone (200 feet long), and a jet-cooling zone (90 feet long). The dimensions cited pertain to a typical unit now in operation. The dimensions are stated to illustrate the comparative lengths of the various elements of a typical horizontal continuous annealing furnace. These dimensions may vary as desired or necessary. The exit section of the furnace consists of a horizontal looping unit and tension reel. The strip is supported as it passes through the heat-treating section of the furnace in catenary fashion by individually-motor-driven alloy steel rolls.
Although the above described continuous annealing practice is quite suitable for most grades of steel, problems are encountered in continuous annealing silicon containing electrical sheet steels, which must be annealed at relatively high temperatures, i.e. above 1,400.degree.F, in decarburizing atmospheres. This combination of high temperatures and decarburizing atmospheres in continuous annealing furnaces frequently causes surface damage on the steel due to roll pickup marking. That is, during such annealing practices, oxidizing conditions must be maintained in the atmosphere to remove carbon from the steel. Carbon is removed by oxidation in the form of CO and CO.sub.2 , which are gaseous phases. At the same time, surface iron and silicon are oxidized to solid oxide forms which are retained on the steel surface. During the continuous annealing through the roller hearth, these surface oxide particles are frequently "picked-up" by the roll surface, i.e. transferred from the steel surface to the roll surface. This build-up of oxides on the roll surfaces will then cause indentations or other undesirable marks on the strip surfaces subsequently passing over the rolls.
To obviate the above problem, several developments have been made, which are primarily addressed to providing furnace rolls having refractory surfaces which tends to resist oxide pickup. For example, U.S. Pat. No. 3,751,195, Snow, discloses a roll having a surface of sinter-banded fused silica. Although such rolls have achieved a moderate degree of success, they have not completely eliminated the problem, particularly where the hot sheet steel wraps around a roll under tension. In commercial facilities where such silicon containing steel strip is processing, it is still necessary therefore, to employ practices for periodically removing oxide pickup from the rolls, even when refractory roll surfaces are used.
A primary object of this invention is to provide a process for continuous annealing silicon containing sheet steel whereby all roll pickup is virtually eliminated.
Another object of this invention is to provide a simple method for eliminating furnace roll pickup during continuous annealing of silicon containing sheet steel.
A further object of this invention is to provide a slightly modified process for continuous annealing silicon containing sheet steel whereby all roll pickup is virtually eliminated by providing a decarburizing atmosphere which will cause only surface silica to be formed on the steel which will not stick to rolls having a silica surface.